TypeScript: Define a union type from an array of strings

Question

I can\'t be the first person coming across this, but my searches have not turned up any useful leads yet. Would greatly appreciate some expert TypeScript advice.

Say

Answer 1

TypeScript 3.4 added const assertions which allow for writing this as:

const fruits = ["Apple", "Orange", "Pear"] as const;
type Fruits = typeof fruits[number]; // "Apple" | "Orange" | "Pear"

With as const TypeScript infers the type of fruits above as readonly["Apple", "Orange", "Pear"]. Previously, it would infer it as string[], preventing typeof fruits[number] from producing the desired union type.

Answer 2

It can be done but first you need an extra function to help infer the string literal type for the array elements. By default Typescript will infer string[] for an array even if it is a constant. After we have an array of string literal types we can just use a type query to get the desired type

function stringLiteralArray<T extends string>(a: T[]) {
    return a;
}

const fruits = stringLiteralArray(["Apple", "Orange", "Pear"]);
type Fruits = typeof fruits[number]

Since 3.4 you can also use a const type assertion instead of the stringLiteralArray function:

const fruits = ["Apple", "Orange", "Pear"] as const;
type Fruits = typeof fruits[number]